This invention relates to centrifuges especially for the sugar industry. Such centrifuges may be of the continuously operating type or of the batch type. Both types of centrifuges have a rotatable centrifuge member such as a drum or cage supporting a screen and liquid flow-off paths on the radially inner surface of the rotating member facing said screen which rests in contact against said inner surface or against a separate support member. While the invention disclosed herein has particular reference to the sugar industry, it will be appreciated that the invention is not limited to such application.
In centrifuges of the above type, the liquid flow-off paths may be formed as grooves in the inner surface of a rotatable drum. In this case, the screen may rest directly on the radially inner surface of the basket. Alternately, the liquid flow-off paths may be formed by a mesh interposed between the inner surface of the rotating member and the screen proper. The invention is equally useful to either of these structures.
In one type of continuously operating centrifuges, the centrifuge basket and screen are conical. The screen is fixed to the basket at the smaller diameter end of the basket, by securing means, for example, by means of a clamping ring. Such securing means are well known in the art. In the past, this technique for mounting the screen has proved to be satisfactory. However, increases in the diameters of the rotating members, increases in the quantity of material treated, as well as increases in operating speeds exposed the screens of such centrifuges to increasing loads especially at the point at which they are rotating the member such as the centrifuge basket. This increased loading of the screens is particularly disadvantageous if very fine screens having rather small screen holes or slots are employed, since, for reasons of manufacture, the thickness of the screens decreases proportionally with the size or width of the screen holes.
Batch type centrifuges generally have cylindrical rotating members such as drums and mechanical discharging and conveying devices for discharging and conveying the centrifuged material. In such centrifuges, apart from possible corrosion caused by the handling of chemically active products, there are mainly two causes for the wear and possible destruction of the screen. On the one hand, wear and destruction may be caused by compressive stress exerted on the screen under normal working conditions during the centrifuging process by the layer of the product to be centrifuged, or, by the solid matter on the screen, and certain tensile stresses may occur on the screen if the centrifuge basket is exposed to elastic strain under the influence of the normal working load. On the other hand, extra wear and tear occurs during discharge operations. Discharging is effected at a relatively low speed, as compared with the centrifuging, speed, and mechanical equipment such as discharger plows, scrapers or the like are employed for removing the solid matter, for example, the sugar from the screen. During this discharging operation, the cover screen is subjected to shearing forces which are directed in parallel to its surface. These forces tend to expand the screen in the circumferential direction, or alternately, to displace the screen. In both cases, the generally fine meshed and thin cover screen is subjected to stresses which may damage or even destroy the screen.
The mentioned damaging effect due to the stresses on the screen is essentially the same in centrifuges having a backing mesh, as in centrifuges having grooves in the basket wall for liquid discharge. The conditions are also similar in the centrifuges having conical rotatable baskets as well as cylindrical rotatable drums. In each case, the generally fine cover screen is exposed to forces acting substantially parallel to the screen surface.